Checklist in the first half second or so, usually unconcious;
1 Hair, eyebrows, eyes.
2 Cheeks, teeth, smile, chin.
3 Collarbone (yes, collarbone), upper bicep, stomach, lower leg, ankle.
3 Movement, Voice, Intelligence.
Boobs? Maybe if they're eyegrabbing, probably not.
Most of my decision of "attractiveness" comes from collarbone, upper bicep and eyes. Hair and smile are a pretty close second, but my feeling during 3 and 4 are the most important. An annoying voice (or god forbid, laugh) can ruin it for me, as can drivel coming out of their mouth.
Usually the first thought is of the intellectual curiosity type as others have mentioned above, but that's it. Unfortunately, I feel like my list and the lists above are going to make people all that more self-concious.
